As one example of fluid flowmeters, there is an air flowmeter disposed in an electronically controlled fuel injector of an internal combustion engine for an automobile, for example, to measure an intake air amount. Such an air flowmeter is mainly practiced in the thermal type because of a capability of directly detecting a mass flow rate.
Among thermal air flowmeters, particularly a thermal air flowmeter employing a measuring element manufactured by the semiconductor micromachining technology has received attention for the reasons that it can be manufactured at a reduced cost and can be driven with lower power.
One known thermal air flowmeter using a semiconductor substrate is disclosed in JP,A 2002-310762. The technique disclosed in that publication places primary importance to a reduction of variations in manufacturing and reliability against changes over time.
In the disclosed invention, a heating resistor, a temperature compensation resistor, and other resistors used in a bridge circuit are formed of the same resistor material on the same semiconductor substrate in an integrated state.
With that construction, heater control with high accuracy can be performed. Even if the resistance values and resistivities of those resistors are changed with the lapse of time, the extents of the changes can be regarded as being the same because those resistors are formed of the same resistor material, whereby the balanced state of the bridge circuit can be maintained. As a result, stable characteristics can be obtained for a long term.